High efficiency cleaning of rotating filter media

ABSTRACT

In an apparatus containing rotating filter media that removes solids from a fluid stream, the invention provides devices and methods for improved cleaning of the filter media. Solids trapped on the filter media are removed by the application of a pressurized fluid spray system. The spray system has nozzles distributed across a spray arm. The nozzles create a generally elliptical spray fan pattern in contact with the filter media. The nozzles are spaced and angularly oriented on the spray arm so that adjacent spray fan contact areas overlap. This distribution and orientation equalizes the distribution of the washing flow and hydraulic energy over the filter area, thereby maintaining the filter material in a more uniformly clean and effective filtering condition.

This application is a Divisional of Ser. No. 09/528,373, filed Mar. 17,2000, now U.S. Pat. No. 6,447,617.

FIELD OF THE INVENTION

The present invention applies generally to a filter apparatus forremoving suspended solids from a fluid stream that is passed throughfilter material. More particularly, the present invention relates tomethods and apparatus for cleaning the filter material.

BACKGROUND OF THE INVENTION

The invention claimed relates generally to the cleaning of rotatingfilter media commonly employed in the treatment of fluids such as water,wastewater and industrial process streams. Such filters often employtextile cloth membranes of cellulose base material, other natural fibersor synthetic fibers woven or napped into a tight fabric or matting. Thecloth filter material is stretched over large drums or multipledisk-type frames. For non-limiting examples, see U.S. Pat. Nos.4,090,965 and 4,639,315.

Typically, the filter media is placed in the flow path of a fluid streamcontaining suspended solid particles which are to be removed by thefiltering process. The solid particles larger than the openings in thefilter media are retained on the upstream, or influent, side of thefilter media while the remaining flow (the filter effluent) passesthrough. Over time, these solids build up on the influent side of thefilter media and impede the rate of filter effluent that passes through,thus necessitating a cleaning of the filter to remove the solidsbuild-up.

Two common cleaning methods known in the art are forward pressurewashing and reverse flow backwashing. Forward high pressure washing isrequired when normal low pressure backwashing cannot assure ultimatemedia cleanliness and when filtration cycles progressively shorten. Apressure washing cycle will reconstitute media cleanliness andreestablish acceptable media headloss. Typically, several backwashevents will occur between pressure washing cycles. Each pressure washoperation may require the filter to stop processing influent, whereasthe backwashing operation typically does not. Pressure washing is aprocess that applies a pressurized water spray from a series of nozzlesevenly displaced along a stationary spray arm positioned to span agenerally radial distance across the filter media. Other filter modelsmay operate with fixed filter panels and moving pressure spray arms andbackwash headers.

In the pressure spray applications, the pressurized spray delivered bythe nozzles dislodges the accumulated solids on the filter media in partby overcoming the adhesive force of the solids against the influent sideand dislodging them from the filter media or in part by driving embeddedsolids particles through the filter media into an effluent channel.Therefore, the effectiveness of the cleaning process includes theapplication of a sufficient washing flow volume and a sufficient spraypressure. Ideally the application of the wash flow and spray pressurewould be evenly distributed across the filter media, but inherentlimitations in the current mechanical design of spray arms and nozzleconfigurations and their angular orientation prevent known systems fromfunctioning in this optimized condition. Moreover, it is desirable tominimize the length and frequency of filter cleaning cycles.Consequently, a cleaning process that cleans unevenly or fails toeffectively remove the embedded solids will require more frequentcleaning and will produce less filter effluent.

It is a known problem with current methods of rotating disk filter mediacleaning that the area of the rotating filter media nearest the axis ofrotation is cleaned more thoroughly than the more radially distantareas. This is primarily due to the substantially higher relationship ofapplied wash flow per unit of filter area at the inner portion of therotating filter media in the prior art. It is also a known problem thatthe filter areas which pass directly under the center portion of apressurized spray nozzle are cleaned more thoroughly than the filterareas that pass under the space between nozzle centers, due tosignificant variations in the contact angle of individual jet streamsemitted by the spray nozzles. The present invention provides improvedcleaning methods and structures that overcome these and otherlimitations of current pressurized spray cleaning techniques.

A significant advantage of the present invention over prior art methodsand devices for cleaning filter material is that the method andapparatus of the invention maintains the filter material in a uniformlyclean, and thus, more effective filtering condition for longer periodsof filter operation. This advantage results because the wash flow rateand hydraulic energy per unit of filter area are applied more evenlyacross the filter media, compared to prior art filter cleaning devicesand methods. The invention significantly reduces the number of washcycles required by the filter and, thus, highly efficient filtration isachieved.

It is a feature and an advantage of the present invention to provide amethod and apparatus for cleaning filter media that does not subject thefilter media to harsh operating conditions, thereby extending theoperating life of the filter material.

It is also a feature and an advantage of the present invention that thenozzles on the stationary spray arm rotate the major axis of the sprayfan contact area into an angular relationship with the axis of the sprayarm to create a spray overlap condition.

It is also a feature and advantage of the present invention that themethod and apparatus described can be positioned to apply a pressurizedspray to either the influent side or the effluent side of the filtermedia.

It is also a feature and advantage of the present invention to improvemedia cleaning performance and increase filtered water production andreduce required spray water pressure, energy, washwater consumption,waste washwater volume and overall cost.

Definition of Terms

The following terms are used in the claims of the patent and areintended to have their broadest meaning consistent with the requirementsof law:

filter media— any permeable material, including but not limited tonatural or synthetic fiber based, granular or membrane compositions;

nozzle— any device or orifice type opening which disperses wash waterfrom a spray arm;

concentric band— a generally circular geometric strip having a width ofa radial distance;

radial distance— a length measured along a straight line whichintersects the center of a rotating filter element; and

spray arm— any conduit for transmitting pressurized wash water to aspray nozzle or a plurality of spray nozzles.

Where alternative meanings are possible, the broadest meaning isintended. All words in the claims are intended to be used in the normal,customary usage of grammar and the English language.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic vertical sectional view through a typicalrotating disk filter apparatus, with parts broken away to illustratedetails of construction.

FIG. 1A is a lateral view of the filter of FIG. 1.

FIG. 2 is a diagrammatic view of a filter media showing a geometricdivision of the filter media into concentric bands of generally equalsurface area.

FIG. 3 is a diagrammatic view of an involute shaped spray arm, withequal spacing of the nozzles on the spray arm.

FIGS. 4 and 4A are diagrammatic top and profile views of a spray nozzleshowing a typical elliptical spray pattern having a minor axis and amajor axis.

FIG. 5 is a sectional view of two adjacent spray nozzles taken alongsection line 5—5 of FIG. 2, showing a spray overlap between adjacentnozzle centers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Set forth below is a description of what is currently believed to be thepreferred embodiment or best example of the invention claimed. Futureand present alternatives and modifications to this preferred embodimentare contemplated. Any alternatives or modifications which makeinsubstantial changes in function, in purpose, in structure or in resultare intended to be covered by the claims of this patent.

FIGS. 1 AND 1A depict a filter apparatus 11 that applies the principlesof the present invention to a known device. The filter apparatus 11 isparticularly applicable to wastewater treatment. The filter apparatus 11includes a filter tank 13 having an influent inlet 15 and an effluentoutlet 17, filter frames 19 disposed between the influent inlet 15 andthe effluent outlet 17, rotating filter 20 and filter media 21 supportedby each filter frame 19. In alternative embodiments, the filterapparatus 11 may include additional filter frames or a single filterframe. In addition, filtering may occur from the outside of the framesinto the center or in the reverse direction.

The filter apparatus incorporates a cleaning system that includes a highpressure spray assembly that is operable to direct a high velocityliquid stream to the surfaces of the filter material. The high pressurespray means includes a high-pressure pump 23 that is connected to a setof spray nozzles or spray heads 25, which are attached to the spray arm24. Each spray head 25 is positioned at a specific distance from aninfluent surface of the filter media 21. When operated, thehigh-pressure pump 23 delivers a liquid stream at high pressure throughthe spray arm 24 to each of the spray heads 25, and each spray head 25directs a high velocity liquid stream against a portion of the surface21. The liquid stream acts to wash the influent surface 21 and to removesolids that have accumulated thereon. The liquid stream also penetratesthe influent surface to impact and dislodge filtered solids entrainedwithin the filter material. In alternative arrangements, the spray headsare positioned inside the effluent chambers to direct liquid streams inopposite directions against portions of the effluent surfaces.

Prior to operation of the high pressure spray cleaning cycle, the liquidlevel in the filter tank 13 is preferably lowered to a height below thespray heads 25. Typically, the filter operation is stopped andsufficient liquid volume is drained from the filter tank to expose thespray heads 25. By exposing the spray heads 25, the high pressure spraymeans is operated more efficiently and more effectively. The filterframes 19 and hence filter media 21 are then rotated until the entirefiltering area has passed under the spray heads 25.

The problems of the prior art pressurized spray filter cleaning methods—uneven distribution of wash flow per unit of filter surface area anduneven distribution of hydraulic energy per unit of filter surface area—have independent solutions that are not readily compatible. Correctingthe distribution of wash flow per unit area can be accomplished byrepositioning the nozzles 25 on the spray arm 24 and by adjusting theirangular orientation.

One method, illustrated in FIG. 2, involves dividing the operativefilter area 21 into concentric bands 30 of equal surface area (A_(b)),the number of bands 30 corresponding to the desired number of nozzles(n) 25.

The selection of a desired number of nozzles 25 is a matter of designpreference. While any number of nozzles 25 may be selected, theimprovements in cleaning efficiency are achieved to a greater degree asthe number of nozzles 25 increases.

Since the concentric bands 30 have equal surface area, they have agenerally decreasing radial width from the innermost to the outermostband 30 to compensate for a generally increasing band circumference.Therefore, when using a single nozzle type for all positions on thespray arm, the major axis 51 of the spray fan 31 must be equal to orlarger than the innermost band width.

Choosing a nozzle 25 with a spray fan major axis 51 (FIG. 4) that islarger than the width of the innermost band 30 provides greaterflexibility in the selection of the degree of overlap desired betweenadjacent spray fans 31. Alternatively, similar flexibility may beachieved by using different nozzle types having larger spray fan axes atthe inner bands and smaller spray fan axes at the outer bands.

Given a known number of bands 30 and a constant surface area per band,the diameter (D_(n)) and width (b_(n)) of any particular concentric band30, can be determined by the following algorithms:${{E\quad {q.\quad 1}}:{\underset{n = 1}{\sum\limits^{n\quad \max}}D_{n}}} = \left\lbrack \frac{A_{2} + {n\quad A_{b}}}{\left( {\pi/4} \right)} \right\rbrack^{1/2}$where:D_(n) = the  outside  diameter  of  the    nth  concentric  band;A₂ = inside  disk  area  (non-filtering)A_(b) = filtering  area  per  nozzle

${{E\quad {q.\quad 2}}:{\underset{n = 1}{\sum\limits^{n\quad \max}}b_{n}}} = \frac{D_{n} - D_{i}}{2}$where:b_(n) = the  width  of  the  nth    concentric  band;D_(i)=  D_(n − 1)=  the  inside  diameters  of  the    nth  concentric  band

Spray nozzles 25 with the same flow and pressure rating are then spacedalong the spray arm 24 so as to be located in the middle of eachconcentric band 30. If the spray arm 24 is oriented along a radius ofthe rotating filter 20, the nozzles are then rotated according to aprimary angle (X_(n)) 61 so that the major axis 51 of the fan spraycontact area 31 for each nozzle 25 spans at least the distance betweenthe inner and outer boundaries of the associated concentric band 30.

Because of clearances required between the spray arm and the tank watersurface during pressure spray operation, and because this tank waterlevel must provide the gravity driving head for the waste wash waterflowing through the filter media and the effluent conduits, such waterlevel is commonly higher than the horizontal center line of the filterdisks. It is, therefore, commonly desirable to have the spray arm 24 ina non-radial relationship to the rotating filter 20 (FIG. 2). If thespray arm 24 is offset parallel to a radius of the filter, the nozzlesmay remain centered on the corresponding radial bands determined byEquation 1, but an adjustment to the angular orientation must be madesuch that it consists of a primary angle (X_(n)) 61 and a secondaryoffset angle (Y_(n)) 60 according to the following equations:

Primary Angle X _(n)=COS^({circumflex over ( )}−1) (S _(n)) where S _(n)=[b _(n)+2b _(n+1) ×OL]/H  Eq. 3:

Secondary Angle Y _(n)=SIN^({circumflex over ( )}−1)(P _(n)) where P_(n) =L _(O) [R _(n)−(b _(n)×0.5)]  Eq. 4:

where:

L_(O)=parallel offset distance of spray arm 24 from radial position

R_(n)={fraction (D_(n)/2)} which is equal to the radial length at theouter boundary of the nth concentric band 30

b_(n)=the radial width of the nth concentric band 30

OL=spray fan contact area overlap 36, expressed as a percentage of thedistance between adjacent nozzles 25 as measured along the axis of thespray arm 24

H=the length of the major axis 51 of the spray fan contact area 31

In an alternative to the preferred structure and method, the spray arm24 can be of an involute shape, as shown in FIG. 3, which would allowfor nearly equal spacing of the nozzles 25 on the spray arm 24. Inanother alternative to the preferred structure and method, the nozzles25 can vary in size and in shape so that the dimensions of the contactarea under the spray fan area 31 better match the varying dimensions ofeach radial band of the filter media and its overlap requirements.

Following the flow distribution corrections for the high pressure sprayapplications described above, the hydraulic energy applied to the filtermedia 21 remains unbalanced due primarily to the varying array ofcontact angles 52 (FIG. 4A) produced by the nozzles 25. In theseapplications of the invention, compensating for the uneven distributionof hydraulic energy across the nozzle contact area 31 requires aconversion of the actual flow into an expression of cleaningeffectiveness, herein referred to as the “effective flow”, thatexpresses the work performed by the applied flow on the filter media 21.In most applications, the nozzles 25 used on a spray arm 24 forpressurized cleaning of filter media 21 have a fan shape contact area 31that induces spray contact with the filter media 21 in a generallyelliptical pattern. FIGS. 4 and 4A show a typical elliptical spraypattern having a minor axis 53 and a major axis 51. The trajectory ofthe jet stream 56 in relation to the plane of the filter media 21defines a contact angle A 52. The contact angle A 52 varies across thecontact area having its highest (most nearly vertical) values in thevicinity of the minor axis 53 and having its lowest values (most nearlyhorizontal) in the outer portions of the major axis 51. The highercontact angle 52 applies greater hydraulic energy to the filter mediaand a corresponding greater cleaning power, particularly for drivingdeep seated solids through the media. Therefore, in a situation wherethe flow distribution is nearly equalized with a minimum of sprayoverlapping, the cleaning power of the spray arm 24 remains out ofbalance due to the variations in hydraulic energy across the contactarea 31 of each nozzle 25.

To correct for this cleaning energy imbalance, the actual flow in theoverlapping contact area must be raised beyond a mere hydraulic flowequilibrium to a point of overcompensation when compared to the flow atthe center segment of the nozzle 25. This overcompensation is bestrelated to the center nozzle flow by a power factor (P), the product ofwhich defines the effective flow. The power factor is expressed as the“sine” of the mean contact angle 52, (P=SIN [A]) within a givenoverlapping spray contact area 31 between adjacent nozzles 25. Theeffective flow range simulates a closer power relationship to that ofthe nozzle center segment and allows for more convenient backwashcleaning adjustments for difficult filtering conditions.

Because of the configuration of the typical nozzle 25, the centersegment 54 typically exhibits the highest actual flow and highestcontact angle. Accordingly, the other segments 55 must be normalizedagainst the characteristics of the center segment 54. The normalizationprocess involves increasing the flow against the band area in the outersegments of each spray fan by overlapping the coverage area of adjacentnozzles 25.

Without the power factor application in the preferred embodiment, a mereequalization on the basis of flow only (one that accounted for thevariation in flow along the major axis of each nozzle segment) wouldindicate an overlap requirement of 30 to 35%. In some applications ofthe inventions, however, this overlap would not achieve a uniform filtermedia cleaning that is optimally desired.

Taking variations in flow and contact angle 52 across each nozzle 25into account, and applying the power factor, the effective flow isoptimized across adjacent nozzle contact areas when the overlap 36 istypically within the range of approximately 40% to 100% of the radialprojection 60 between adjacent nozzle centers (FIG. 5). A percentilespray overlap 36 from 40 to 100 percent of the radial projection 60between adjacent nozzle centers covers an adjustment range typical forwater filter applications

It will be apparent to one of ordinary skill in the art thatequalization of effective flow could alternatively be achieved in someother overlap range with variations in nozzle design or quantity. Sincenozzle design adjustments are foreseeable, it is expected that thelimits of the overlap ranges may also vary. However, the process ofdeveloping power factors based on the contact angle with an objective ofcreating a generally uniform distribution of cleaning effectivenessacross a rotating filter, will reflect an application of the describedmethod.

Generally, the inventions described herein provide systems and methodsfor improving the efficiency of cleaning rotating filter media 21. Therotating media 21 is divided into concentric bands 30 of approximatelyequal surface area having decreasing width and increasing circumferencefrom the innermost to the outermost bands 30. Nozzles 25 are chosen bythe user of the invention so that the spray fan area 31 generatedagainst the filter media 21 has a major axis 51 at least as wide as theinnermost band 30. The nozzle 25 is positioned in the middle of eachconcentric band 30. The major axis 51 of each spray fan area 31 isoriented in an angular relationship to the spray arm 24 so that adjacentspray fan tips cover an overlapping area of the filter media 21. Anoverlap arrangement that generally spans 40% to 100% of the radialdistance between adjacent nozzles 25 achieves an equalization of appliedwash water flow and hydraulic energy per unit of filter area so as toachieve a highly efficient cleaning of the filter media 21. In thisimproved condition, the filter is maintained in a more uniformly cleanand effective filtering condition for larger periods of time thanconventional techniques.

The above description is not intended to limit the meaning of the wordsused in the following claims that define the invention. Rather, it iscontemplated that future modifications in structure, function or resultwill exist that are not substantial changes and that all suchinsubstantial changes in what is claimed are intended to be covered bythe claims.

What is claimed is:
 1. An apparatus for cleaning rotating filter mediaby pressurized spray comprising: at least one spray arm in communicationwith a source of pressurized water; a plurality of spray nozzles affixedto said at least one spray arm, the flow from each of said nozzlesdefining a generally elliptical contact area on said filter media, thespacing of said nozzles being non-uniform and the orientation of themajor axes of said contact areas being not colinear; and, said nozzlesoriented on said at least one spray arm such that a portion of thefilter media passing between adjacent nozzles receives a pressurizedspray applied from each of said adjacent nozzles.
 2. The apparatus ofclaim 1 wherein said portion of the filter media receiving a pressurizedspray applied from each of said adjacent nozzles has a radial widthbetween approximately 40% and approximately 100% of the radial distancebetween the centers of any two adjacent nozzles.
 3. The apparatus ofclaim 1 wherein the spacing between the nozzles represents a geometricdivision of said filter media into concentric bands of approximatelyequal surface area with the center of each nozzle located in the middleof a concentric band.
 4. An apparatus for cleaning rotating filter mediaby pressurized spray comprising: at least one spray arm in communicationwith a source of pressurized fluid, said spray arm having a generallyinvolute geometric shape; a plurality of spray nozzles affixed to saidat least one spray arm, the flow from each of said nozzles defining agenerally elliptical contact area on said filter media, the orientationof the major axes of said contact areas being not colinear; and, saidnozzles oriented on said at least one spray arm such that a portion ofthe filter media passing between adjacent nozzles receives a pressurizedspray applied from each of said adjacent nozzles.
 5. The apparatus ofclaim 4 wherein said portion of the filter media receiving a pressurizedspray applied from each of said adjacent nozzles has a radial widthbetween approximately 40% and approximately 100% of the radial distancebetween the centers of any two adjacent nozzles.
 6. The apparatus ofclaim 4 wherein the spacing between the nozzles represents a geometricdivision of said filter media into concentric bands of approximatelyequal surface area with the center of each nozzle located in the middleof a concentric band.